The invention relates to interconnecting semiconductor devices to fabricate hybrid integrated circuits.
Hybrid integrated circuits are an important class of circuits in which two or more devices, or possibly dissimilar materials, are interconnected to perform a function that is not feasible with a monolithic device. Typically, connections are made to the devices using wire bonding to pads arranged around the perimeter of the device. Other methods such as automated tape bonding and beam leads, are also limited to contacts being made to the periphery of the device.
For some time it has been realized that applications exist for which the method of attaching leads around the edge of the device is inadequate. Primary examples are imaging arrays, in which a photosensitive array is bonded to a second device that performs amplification and readout functions, and random logic functions in which the number of signal leads are too large to be placed all along the perimeter of the device. The most common approach to imaging arrays is the use of indium bump bonding, which does not require the use of elevated temperatures. For random logic functions applications the technology of solder bumps has been widely employed.
Hybrid imaging arrays are an important use of indium bump bonding for medium-and far-infrared detection, as well as potentially the hard x-ray and gamma-ray region. In these portions of the spectrum (.lambda.)1000 nm and .lambda.(0.1 nm, respectively) silicon itself has no useful response, while the materials that are sensitive in these regimes (HgCdTe, InSb, CdTe, Hgl etc.) lack a well-developed and high-yield technology for integrated readout circuitry. Although it has been in development for many years, the indium bump-bonding approach is thought to be expensive and difficult to perform, and for large arrays involving thousands of bumps the probability is very high that at least a few bumps will fail to make electrical connection.
Another approach to the problem is to grow the radiation-sensitive material directly on the silicon readout chip. This has several problems, such as compatibility between the epilayer growth conditions and the silicon device, the generally inferior quality of heteroepitaxially grown material compared to homoepitaxially or bulk-grown material, and limitations on the practical thickness of the epitaxial layers. The latter is particularly important for hard x-ray and gamma-ray imaging, where the detector material may need to be hundreds of .mu.m thick for useful sensitivity.